In recent years, high-speed continuous shooting or movie shooting functions are mounted and become popular as standard functions in digital cameras ranging from compact cameras to single-lens reflex cameras.
As a method for quickly reading out image data of an imaging element having a large number of pixels, column-parallel AD conversion method is generally used. The column-parallel AD conversion method is intended to incorporate an ADC for each of columns of a pixel array, and perform AD conversion at one time on the signals output from pixels in the column of the pixel array in a horizontal scanning period. As an ADC circuitry in this case, a single-slope type ADC (hereinafter denoted as an SS-ADC) is generally used. The SS-ADC is a small-scale circuit that can be housed in an area having a width corresponding to a column width that is determined by a pixel pitch. The SS-ADC is intended to input, as a reference signal, a ramp signal having a correlation with a count value of a counter to a comparator, and output, as a result of analog-to-digital (AD) conversion, a count value at the time when a match between an analog signal and a reference ramp signal is detected by the comparator.
With the increase in the resolutions of solid-state imaging elements, noise reduction is desired. In general, noise that occurs from a solid-state imaging device can be roughly divided into fixed pattern noise (FPN) and random noise. As for FPN, a column in which noise occurs is fixed on a per device basis. Thus, noise is optimized on a per device basis according to a correction technique by digital signal processing performed at a subsequent stage of the solid-state imaging device, and a large part of the noise can be removed. As for random noise, it is difficult to perform such a correction process. However, the random noise appears at random on the entire image generally according to a normal distribution, and thus it is difficult to visually recognize the noise. However, random noise components that are generated mainly from noise superimposed on a reference ramp signal of the SS-ADC appear in irregular rows and levels. Accordingly, random noise components have features that they appear in a vertical direction of the image without according to a normal distribution, and are noticeable as random horizontal noise.
As a configuration of a ramp generator circuit, a ladder resistor type voltage DAC, a current-steering type DAC or the like is used (see Patent Literature 1 and Patent Literature 2).
In addition, as a method for increasing the slope of a ramp signal voltage that is a time variation rate of the ramp signal voltage, the slope can be increased by increasing the difference between a start voltage and an end voltage of a ramp signal, or alternatively by increasing a driving frequency of the ramp signal (see Patent Literature 2).